Immunofluorescent techniques have been developed to detect viral antigen in situ by cryostat sectioning of skin biopsies taken from mice previously infected with LaCrosse (LAC) virus. Large amounts of viral antigen were detected in a variety of suckling mouse tissues. In older mice (3 week old) antigen was restricted to one or two tissues. The actual tissue tropisms will be determined using histological staining of alternate cryostat sections as well as by electron microscopy. The biopsy technique was used to examine 8 biopsies obtained from patients with suspected LAC virus infection. Antigen was detectable in 4 of 8 sections, but only in low levels, possibly due to freezing the biopsies in isopentane before mailing to Yale. The technique will be applied in the hospital setting in LaCrosse, Wisconsin to help diagnose cases during the 1979 epidemic season. Results will be compared and contrasted with normal diagnostic procedures. Blood and CSF leukocytes will be examined for the presence of viral antigen. Serum and CSF will be examined for early appearing specific antibody by both a "spot-slide" technique or by an ELISA (enzyme linked immunosorbent assay) technique. Radioimmunoassay will be developed as the standard of sensitivity for assessing other techniques.